Conventionally, a technique to adjust acoustic characteristics of a headphone by forming an opening in a housing of the headphone is known. Japanese Unexamined Patent Application Publication No. 2009-033768 discloses a technique that improves acoustic characteristics by providing a housing with a port having (i) a through hole closed with an acoustic resistance material for communicating between a rear cavity of a headphone and the outside and (ii) acoustic mass.
In the conventional technique, acoustic characteristics can be adjusted by adjusting the diameter and the length of the port. However, there were problems that adjusting acoustic characteristics by changing the diameter and the length of the port required many man-hours and that finely adjusting acoustic characteristics was difficult.